The term “metal finishing” generally refers to the application of a desired treatment, texture, or coating to the surface of a metal. Metal finishing processes dedicated to the coating of aluminum metal objects are intended to lend aesthetics, utility, and corrosion protection to the coated metal surface. Desired attributes of metal coatings include good adhesion to the metal surface, coating color retention, wear resistance, and corrosion protection.
A variety of non-paint processes are commercially available for blackening aluminum and aluminum alloys. The most prevalent process is anodizing, an electrolytic immersion process that produces aluminum oxide coatings of varying colors, including black. Up to 21 process steps are required to produce anodized coatings. While anodized coatings are extremely durable and corrosion resistant, the application of such coatings is generally restricted to rack work and is not suitable for parts that could otherwise be barrel or basket loaded for greater throughput efficiency. Care must be exercised in racking parts for anodizing to avoid rack marks resulting from poor contact of the aluminum surface with the anodizing solution.
Another process, referred to as “cold blackening,” involves the deposition of copper selenide coatings from aqueous media on steel and aluminum substrates. An example of this process is described in U.S. Pat. No. 2,303,350. The aqueous solution contains selenium and copper salts and is applied at room temperature by brush, wipe-on and immersion techniques using 7 to 8 process steps.
The coatings produced by the “cold blackening” process offer good color stability, but are relatively non-adherent and therefore exhibit poor wear resistance. As such, a sealant, wax or other corrosion protective coating must generally be applied immediately following the blackening process to secure the coating and protect the metal surface. In addition, the selenide coating itself affords no corrosion protection to the metal. Finally, this process presents disposal issues as selenium can be highly toxic at even moderate concentrations. The U.S. Environmental Protection Agency has set a maximum contaminant level (MCL) for selenium of only 0.05 parts per million.
Black coatings on aluminum can also be formed by contacting the metal surface at room temperature with a sodium hydroxide solution containing zinc and antimony compounds. However, this process requires tight temperature control as higher temperatures will promote undesirable surface etching. Prolonged immersion times in the blackening treatment also result in rough, non-adherent coatings. Finally, this process presents disposal issues as antimony can be highly toxic. The U.S. Environmental Protection Agency has set a maximum contaminant level (MCL) for antimony of only 6 parts per billion.
As such, a need remains for methods and compositions for coating aluminum substrates.